The axons, dendrites, and synapses of individual neurons must be positioned at appropriate targets to establish and maintain functional neural circuits. Our long-term goals are to elucidate how cellular signaling establishes neural circuits, to understand how cellular communication translates into neural morphogenesis, and to dissect how disruption of neural circuit assembly may result in impairments in neurodevelopmental disease. A core principle in brain development is that circuit assembly and neural morphogenesis require spatiotemporal regulation of cytoskeletal remodeling mediated by both intracellular and extracellular signaling. However, while the signaling pathways that guide axonal outgrowth have been extensively characterized, the pathways that direct dendrites into their target fields remain obscure, in large part due to the complicated morphology and small size of dendritic branches. Using the Drosophila aCC motoneuron, which has a highly stereotyped, simple dendrite pattern, and molecular marker systems that allow examination of individual cells in complex environments, we have recently gained insight into the specification of dendritogenesis by inter-neuronal interactions. 1) We found that interaction between the aCC and its target neuron (MP1) is mediated by Down syndrome cell adhesion molecule (Dscam1). 2) The Dscam1 receptor recruits the Dreadlocks (Dock) adapter protein and the Pak1 kinase to the membrane in the aCC. 3) Subsequently, Pak1 interacts with activated Cdc42 GTPase, leading to cytoskeletal rearrangements at the contact site. These findings have led to a novel model in which the Dscam1-Dock-Pak1 and the Cdc42 pathways converge to regulate aCC dendritogenesis. Using a combination of genetics, biochemistry and microscopy techniques, the objective of this project is to address key gaps in our understanding of dendrite specification by this signaling pathway. In Aim 1, to define the potential role of the secreted ligand Slit in mediating Dscam1 interactions at the aCC-MP1 contact site, we will examine whether a glycoprotein called Slit facilitates communication between Dscam1 receptors on the aCC and the MP1 neurons. In Aim 2, to elucidate the mechanism by which Dscam1 interactions lead to Y-phosphorylation in the cytoplasmic domain, we will investigate how ligand binding promotes tyrosine phosphorylation of Dscam1 to stimulate Dock binding. In Aim 3, to identify upstream signaling that activates Cdc42 and aCC dendritogenesis, we will examine whether the Ephrin receptor activates Cdc42 at the onset of aCC dendritogenesis via an Eph-interacting guanine nucleotide exchange factor (Ephexin). The proposed studies will provide significant insights into the molecular mechanism of dendritogenesis in the CNS, a critical process that is greatly under-explored. In the long term, these studies will provide a foundation for understanding the etiology and potential treatment of neurodevelopmental diseases.